The purpose of these studies is to evaluate the mechanisms of immunosuppression in mice and humans associated with combined exposures to polycyclic aromatic hydrocarbons (PAHs) and sodium arsenite (As+3). Suppression of the immune system is known to be associated with a decreased ability to fight infections and cancer. Preliminary data is presented that mice demonstrate synergistic immunosuppression when exposed to As+3 and PAHs in vivo through their food and drinking water. We also present preliminary data demonstrating that human peripheral blood mononuclear cells (HPBMC) are immunosuppressed at environmentally relevant levels of sodium As+3 found in drinking water. PAHs were found to increase the amount of As+3 induced immune suppression as well. Therefore, it is important to understand the mechanism(s) of immunosuppression produced by these agents when exposures occur alone or in combination. The central hypothesis to be tested in this application is that PAHs and As+3 produce synergistic immunosuppression through DNA- damaging and repair pathways. It is important to develop animal models to study synergistic immunosuppression and to pursue mechanistic studies of relevance to human exposures. These studies will determine whether co-exposures from As+3 in drinking water and PAHs in the diet (which are present in the air we breathe and the food we eat) in mouse models result in greater suppression than has previously been found for either class of agents on their own. We will determine the biochemical mechanisms potentially responsible for these interactions, and we will determine whether Zn+2 present in drinking water can protect mice from As+3 exposures. Finally, because we found that human blood T lymphocytes are extremely sensitive to low concentrations of As+3 that are present in drinking water in many populations in the U.S. and elsewhere in the world, we will determine the sensitivities of various humans (males and females of different ages and ethnicities) to these exposures. We will obtain peripheral blood cells from these individuals to determine whether their immune response are suppressed by As+3 and PAH given in vitro alone or in combination. The results of these studies will make an important contribution to understanding environmental agents that modulate the human immune system and perhaps provide an approach to intervention for As+3 drinking water exposures. PUBLIC HEALTH RELEVANCE: The purpose of these studies is to evaluate the mechanisms of immunosuppression in mice and humans associated with combined exposures to polycyclic aromatic hydrocarbons (PAHs) and sodium arsenite (As+3). Suppression of the immune system is known to be associated with a decreased ability to fight infections and cancer. Preliminary data is presented that mice demonstrate synergistic immunosuppression when exposed to As+3 and PAHs in vivo. We also present preliminary data demonstrating that human peripheral blood mononuclear cells (HPBMC) are extremely sensitive to T cell immunosuppression by environmentally relevant levels of exposure to sodium As+3 and that PAHs interact significantly with As+3 as well. Therefore, it is important to understand the mechanism(s) of immunosuppression produced by these agents when exposures occur alone or in combination. The central hypothesis to be tested in this application is that PAHs and As+3 produce synergistic immunosuppression through genotoxic pathways resulting from DNA adduct formation and inhibition of DNA repair. It is important to develop in vivo models to study synergistic immunosuppression and to pursue mechanistic studies of relevance to human exposures. In SA1 we will utilize our established mouse model for studies of the combined effects of PAHs, on the T-dependent antibody response (TDAR) in murine spleen and pre-B and granulocyte/monocyte (GM) progenitor cell activity of bone marrow (BM) cells. We provide exciting new in vitro data demonstrating that Zn+2 can overcome the immunosuppression (spleen TDAR) produced by exposures to As+3, perhaps based on the zinc finger protein theory discussed below. We know that genotoxicity and p53 signaling are important mechanisms of immunosuppression produced by PAHs, and perhaps for As+3 in mice. Therefore, we will examine the p53- dependence of spleen and BM suppression using p53 null mice. In SA2 we will determine whether the mechanism of synergy is due to inhibition of DNA repair. Our preliminary data show that As+3 inhibits DNA repair and that co-exposures to PAHs and As+3 may lead to an increase in PAH bulky and oxidative stress adducts resulting in increased p53 signaling. In our previous studies we showed that As+3 binds to the zinc finger proteins, PARP-1 and XPA, leading to inhibition of DNA repair in both Nucleotide Excision Repair (NER) and Base Excision Repair (BER) pathways. We will test the hypothesis that Zn+2 supplementation can reverse the effects of As+3 in vivo in mice. In SA3, we will build on our preliminary data that human T cells are inhibited by low nanomolar (nM) concentrations of As+3 exposures in vitro. We will also examine the ability of Zn+2 to reverse this immunosuppression in HPBMC in vitro. Because we have found that there are significant inter-individual differences, we will perform initial characterization of As+3 and PAH exposure in 150 different individuals. Completion of these studies will yield critical new data relating to our central hypothesis and will determine th potential sensitivities of the human immune system to binary exposures of PAHs and As+3.